Instructions
Initial Reply due by Thursday Feb 20th at 11:59pm | Second Reply due by Sunday Feb 23rd at 11:59pm
This week's discussion concerns the second law of thermodynamics. Let's start by considering a broad definition of the second law: 𝑑𝑆>𝛿𝑄𝑇, where dS is some infinitely small piece of Entropy of the system, dQ is an infinitesimally small piece of heat being added to a system, and T is the temperature of the environment outside of the system. Okay- then what does that equation mean? It means that when taking thermal energy from the environment, and adding it to the system via heat, the disorder of the system increases more than the energy that was added. This is very similar to the definition given in Ch 20, by implying a lack of perfect efficiency when transforming heat into mechanical work.
A
A much more general interpretation of this definition is that nature tends towards increasing entropy, and it requires more work to undo an entropy increase than it takes to cause an entropy increase. When worded this way, we can observe macroscopic examples of this second law applying to our lives.
- Think of a macroscopic example of the second law of thermodynamics that affects you in your daily life.
- Some examples, because I know this is a bit of stretch in getting from the second law to macroscopic real life situations:
- Consider a Lego set. The builder expends a bunch of time/energy building the set. By building the set they enforced a specific location upon each brick, reducing the entropy of the set as a whole. When disassembling the set, they spent (probably) less energy to undo all of that work. This is an example of a system requiring less energy to increase entropy than is needed to decrease entropy.
- When burning a candle, a bunch of organic compounds are undergoing a chemical reaction of burning. When the wax is in its solid form as a candle it is fairly low in entropy since all of its molecules are bound in a single block. Once burning, however, the products of the chemical reaction are flung in to the atmosphere as gasses. These gasses mix with the local environment, and have a much higher disorder than they had before burning. The input energy is just the tiny flame that burns the candle, and it causes a huge amount of entropy increase! Imagine how hard it would be to collect back all of the molecules that were flung in to the air, reverse the chemical reaction, and then pack it all back in to a candle shape!
- An example from a former student: I spent 30 minutes cleaning and organizing the kids' room, they only needed 5 minutes to make it just as messy again.
- Additional shorter examples:
- mixing of things, chemical reactions, phase changes (entropy increases solid>liq>gas, decreases in the opposite direction), expansion of gases, any time thermal energy spreads out, natural decay, both in the radiation sense and nature sense, any biological process
- Yes, you can use the above (in your own words) if they affect you.
- Some examples, because I know this is a bit of stretch in getting from the second law to macroscopic real life situations:
- Once you have your topic, briefly share that example here. Please include the following in your reply:
- Explain how exactly entropy is increasing or decreasing, and the relevant heat or work inputs/outputs of the system.
- This initial reply is worth 5 points.
- Reply to at least one other student to discuss their example.
- Please pick a post on a different example than the one you chose.
- Describe how you have experienced the process they describe yourself, or try to imagine up a way in which their example can be used or useful in general.
- Basically just try to expand on their post and add to it.
- This second reply is worth 5 points.
- Ensure you are subscribed to the discussion, so you remember to come back and view the replies to your post!
Submission
To reply to the discussion, click "Reply" on the main instruction page and type your paragraph into the text submission box. To post your video, click the "Links" button above the text box. Once you are done forming your response, click "Post Reply."
Grading
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